The central theme of this Program is improving gene transfer and gene correction in human hematopoietic stem and progenitor cells to provide more effective and safe therapies for blood cell disorders. While the concept of gene therapy using HSC to provide improved methods to treat congenital disorders has been under study for at least two decades, there have been only a small number of successful clinical applications. The techniques currently in use for gene transfer and expression in HSC are sub-optimal in most cases to yield clinical benefits. The goals of this Program are to investigate the mechanisms limiting successful clinical applications of gene transfer and to develop improved techniques which will broaden the range of diseases which may be treated effectively and safely. These advances can only be realized by combining each of these individual projects into a unified Program. This Program has three projects: 1. Selective Expansion of Hematopoietic Stem Cells and Lymphoid Progenitors, 2. Genome Modification in Human Hematopoietic Stem cells, and 3. Gene Therapy for ADA-deficient SCID. Four Cores (Administrative, Cell Isolation and Analysis, Vectors and Animals) will support the projects with integrated services for optimal quality and efficiency. The Project and Core leaders have complementary expertise in the relevant areas of experimental hematology, gene therapy, immunology, and signal transduction and have a long-standing record of interactive collaborations.